1. Field of the Invention
This invention relates to a process for the alkylation of an aromatic hydrocarbon by reaction with an olefin in the presence of ZSM-23 catalyst.
2. Description of the Prior Art
Alkylation of aromatic hydrocarbon compounds employing certain crystalline aluminosilicate zeolite catalysts is known in the art. For instance, U.S. Pat. No. 3,251,897 describes liquid phase alkylation in the presence of crystalline aluminosilicates such as faujausite, heulandite, clinoptilite, mordenite, dachiardite, zeolite X and zeolite Y. U.S. Pat. No. 2,904,607 shows alkylation of hydrocarbon compounds in the presence of crystalline metallic aluminosilicates, such as, magnesium aluminosilicate. The use of certain shape selective catalysts which have improved aging properties is taught in U.S. Pat. No. 3,751,506. U.S. Pat. No. 4,107,224 is specifically drawn to the manufacture of ethylbenzene by alkylating benzene with ethylene in the presence of certain shape-selective catalysts such as ZSM-5 under specified reaction conditions.
While the latter type catalysts represent a distinct improvement over previously suggested crystalline aluminosilicate catalysts particularly with respect to improved aging properties, they have the disadvantage of producing unwanted quantities of impurities along with the desired alkyl aromatic product, thereby decreasing the overall yield and selectivity for such product.
Thus, in the alkylation of benzene with ethylene, while desired ethylbenzene is the major product, small amounts of di- and possibly triethylbenzenes are always produced simultaneously with ethylbenzene, such amounts depending on the conversion of benzene to ethylbenzene. The polyethylbenzenes formed can be recycled to the alkylation zone, where they undergo transalkylation with benzene to produce more ethylbenzene. Alternatively, the polyethylbenzenes can be transalkylated with benzene in a separate reactor. The formation of polyethylbenzenes hence does not constitute an ultimate loss of the alkylating agent, ethylene. On the other hand, aromatic compounds other than ethyl- and polyethylbenzenes, that are formed during the alkylation reaction generally referred to as by-products, result in an irreversible loss of ethylene and cause difficulties in the product purification. By-products produced during ethylation of benzene include, for example, toluene, xylenes, cumene, n-propylbenzene, ethyltoluene, butylbenzene and other C.sub.10.sup.+ aromatics, the majority being C.sub.7 -C.sub.9 aromatics. The formation of these by-products is increased when the benzene conversion to ethylbenzene is high.
C.sub.9.sup.+ aromatic by-products from the alkylation of benzene with ethylene in the presence of shape-selective catalysts such as ZSM-5 zeolite catalysts, result from the formation of transalkylation intermediates. Such transalkylation by-products which include polyethylbenzenes and polycyclic aromatics, not only degrade the ethylbenzene product purity but accelerate the catalyst aging rate as well when these by-products become part of the recycle feed to the reactor.
In the past, efforts have been made to prevent the formation of the relatively bulky transalkylation intermediates by reducing the effective pore size of the ZSM-5 zeolite. U.S. Pat. No. 3,906,054, for example, discloses a method for reducing the effective pore size of ZSM-5 zeolite by incorporating a small amount of phosphorous with the crystal structure.